Many common electronics applications today utilize mixed signal electronics with Digital, Analog, and Radio Frequency devices and circuits contained on a single functional board or module. Successful operation of such electronics requires that various components be adequately cooled to prevent overheating, and while still providing adequate shielding to prevent Electromagnetic or Radio Frequency Interference (EMI/RFI).
These two requirements post significant challenges when frequencies and power densities of individual components in the module are mixed. In many cases, some devices may preferably operate at frequencies that are orders of magnitude different than others. For example, today's computers are operating at clock speeds at frequencies beyond 1 GHz, while bus speeds and memory speeds are on the order of 100 MHz and local switch mode power converters or regulators operate in the KHz or low MHz range. The power densities of various components can also vary by orders of magnitude, with high power RF amplifiers or advanced microprocessors in the 100 W/cm2 range, while other devices operate on the order of 1 W/cm2.
The traditional method for packaging and cooling such modules is to utilize two-dimensional, planar packaging with forced air convection and conduction cooling methods. Low power density circuits are typically cooled by forced air convection where the thermal path is through the top sides of the component packages. High power devices typically utilize attached heat sinks. In mixed signal applications, different parts of the circuit must be isolated from each other to prevent Electromagnetic or Radio Frequency Interference (EMI/RFI) from adversely influencing the circuit performance. One conventional approach for example for accomplishing this is to segment the circuit card into sections bordered by a ground strip and place a cover containing multiple cavities on top of the board to mate with the ground strips to create a multiple cavity isolated board. This prevents the use of conventional forced air cooling approaches as described above because no air can be circulated through adjacent cavities without violating the effectiveness of the shield. Consequently, most mixed signal modules are cooled by conducting heat from the devices, through the board and into a finned metal heat sink. Higher power devices are typically mounted directly to the heat sink by making a rectangular “hole” through the board.
This traditional method of packaging and cooling is relatively expensive, yields lower reliability, and will be increasingly difficult to effectively utilize in future systems as frequencies and power densities increase. There are three reasons that this approach is expensive. First, the large board sizes utilized to accommodate multiple circuit types/functions on a single card are very complex and more expensive to manufacture. Next, the packaging utilized at the device level to reduce thermal resistance through the card or into the heat sink is expensive because of the requirement to use high conductivity materials with matched coefficients of thermal expansion. Finally, the mechanical parts are complex and require high manufacturing tolerances and complicated assembly. Many of the devices utilized are actually designed to be cooled from the top side, not through the device package and the board. The thermal path through the card has high thermal resistance, consequently yielding high junction temperatures and poor reliability. As these systems move towards the increased use of high performance digital electronics, this method of packaging and cooling will be increasingly difficult to accommodate without further exacerbating the reliability problem because of the trend of increasing power density at the component level.
To alleviate these problems, a new cooling technology which provides the electronics with dielectric fluid has been proposed and described in several patents. The apparatus and method described in U.S. Pat. No. 5,675,473 illustrates the introduction of spray cooling into a traditional multi-cavity, two-dimensional board type of packaging approach described above. While this packaging and cooling approach will indeed provide shielding and improved reliability due to reduced temperature operation as described, this approach will be difficult and costly to implement in practice.
A reason that this method will increase cost is that spray cooling requires the volume flux of spray applied to the electronic components to be matched to the heat flux distribution of the components. Otherwise, cooling performance and device reliability are compromised. Proper cooling is only achieved if a thin liquid film is maintained over the device. If there is too little flow, the liquid layer covering the electronic component will dry out and cause the component to overheat. If the flow to the component is too high, the device will become flooded, and this may reduce the cooling efficiency. Vapor generated at the surface of the component cannot escape effectively and could result in a boiling heat transfer failure mode called burnout.
Even when the volume flux of coolant is properly matched to the heat flux of the device, the excess fluid sprayed within a cavity must be managed, for example, by the method described in U.S. Pat. No. 5,220,804, which prevents or reduces the overflow from adjacent components from interfering and causing flooding type failure conditions.
Thus, the method proposed in U.S. Pat. No. 5,675,473 which describes a single manifold plate that incorporates all of the fluid distribution and return passages and spray hardware would be relatively costly and also exhibit lower than desired yield. Any change to any part of the circuit design will require the use of a new plate assembly to accommodate the required changes to the atomizer array and the fluid return passage designs. Similarly, the failure of any part of the design, on either the card or in the fluid distribution plate requires the entire assembly to be replaced. This type of fluid system would also be very difficult to design for effective operation, due primarily to the need to design a properly balanced two-phase fluid distribution system. With all of the different segments supplied via the same manifold any change in flow rate to a given section alters the pressure drops within the distribution manifold, and consequently may change the discharge pressure in adjacent atomizer groups. This also alters the spray characteristics and flow rates. A worse problem may occur on the fluid return manifold side. A high power density cavity with higher flow rate and high vapor mass fraction will yield a high momentum flow in the exit channel. If this exit channel is in direct communication with a lower power density, lower flow rate cavity, the exit flow from the low power cavity may be impeded, and possibly even backflow, resulting in a flooded operating condition.
The present invention is directed towards one of its objectives namely to provide an improved apparatus and method for 3-dimensional packaging and cooling of mixed signal, mixed power density electronic modules.